Generally, the USB I/F is used to connect a USB host with a USB device and to communicate. For example, the USB I/F has spread as a general-purpose I/F that connects a personal computer (PC) as a host with a job processing device like an image forming apparatus (for example, a printer).
Incidentally, communication speeds supported by USB 2.0 are 1.5 Mbps in a low speed (LS) mode, 12 Mbps in a full speed (FS) mode, and 480 Mbps in a high speed (HS) mode. Then, the communication speed is determined according to types of a USB host and a USB device connected to the USB host.
Further, USB 3.0 supports a super speed (SS) mode of which the communication speed is accelerated to 5 Gbps. The SS mode does not only accelerate the communication speed, but also enables the USB device to control a shift to a low power consumption state in the USB I/F communication, which was conventionally possible only from the USB host. Therefore, the SS mode is more convenient for USB device that aims to save power consumption.
On the other hand, the higher communication speed unescapably increases probability of poor communication due to an effect of noise. This may disturb normal data transmission from the USB host normally.
Here, it is assumed that a USB device of the SS mode connection and a USB device of the HS mode connection are connected to a USB host supporting USB 3.0 via a USB hub supporting USB 3.0. In this case, a communication in the SS mode and a communication in the HS mode are performed concurrently between the USB host and the USB 3.0 hub.
Then, the concurrent communications in the SS mode and the HS mode generate crosstalk noise that becomes a new factor to disturb the communications.
In order to prevent the poor communication in the communication using the USB I/F, there is a known technique that detects the poor communication caused during the communications. The technique lowers communication speed to keep a stability of communication when detecting that the poor communication continues beyond a fixed time (for example, see PTL 1).
However, since the technique disclosed in PTL 1 changes the communication speed after detecting the poor communication, it is difficult to prevent the poor communication before changing the communication speed.